Render, Plaster or Board?

[Peter Hemmings]

Hi,

I have a 50's house with solid walls but very dry rendering which tends to have hollow parts on some walls.

I have an imperial bathroom and want to put a metric bath in it! The width of wall between the ends of the old bath were about 25cn too short, so I am at present removing a "slice" from the old breeze blocks from one end (rear of airing cupboard). The old "70's" tiles are being removed together with the original tiles underneath and of course the old rendering!!

I have allowed for 12.5mm aqua panel+glue+tiles at the shower end and would appreciate some advice on what to do at the back and other side of the bath (I have not replaced a bathe for some years).

I intend to make my "chiseling" a bit more even by rendering the worst areas. Years ago I just used a cement render, has bonding/bonding plaster replaced this or should I still render?

On the other walls I will need to have a base for tiling about 5-7mm thick, again should I be rendering or plastering? I assume wall boarding is not a good idea near the bath!

If bonding, do I need to also use a finishing coat, I was think of just tiling on the bonding?

Must you put a finishing coat onto dry or wet bonding (general question)?

I am off to create more black mess in the bathroom now!

Thanks for your time

Peter H

[boltmail]

Render and skim is still used, up to you really.

Render is a much better base for tiling on than plaster (stronger, less chance of adhesive failure).

Tiling onto bonding is a bodge - it's too porous for the adhesive, and too weak. It needs a finish coat, and then if you are using a cement- based adhesive it should be primed with something like the BAL APD, to stop the possibility of ettringite failure. if you are using a ready- mix tub adhesive, it can go on the finish coat.

Either drying, or dry, not wet. If the latter, wetting down helps. Not that I'm much of a plasterer.

[sm_jamieson]

I disagree. Skimming is a waste for tiling over. Seal with PVA, then tile over that. Never caused problems for me. Use render in wet areas, since plaster will fall apart in the damp.

Simon.

[boltmail]

The trouble with pointing out what good practice is that many bodges don't fail and so there is then a chorus of anecdotal evidence that bodges are fine. Check BS5385, look on the Tile Association's website, or any of the adhesive manufacturers; they don't agree with you.

[sm_jamieson]

My advice about not skimming first originally came from this group, and it has saved me loads of dosh, since my skimming is pour, but I just scrape off undercoat against battens. Who decides what is a bodge ? If a bodge never fails, it is probably not a bodge ! (OK, unless it causes some small chance of catastrophic failure) There is no reason that a skim coat will stick to the undercoat plaster any more than the tile adhesive will. And no reason tile adhesive will stick better to skim than to undercoat. In fact, the undercoat gives a better key due to it's roughness, but it should be sealed to reduce suction (just like when you skim anyway). I've seen skim coats that easily flake off the undercoat, and there is no way this would give a better tiling substrate. But the point was, skimming first does not make the tiles any more firmly fixed. Simon.

[Stuart Noble]

The main thing is to have a level, flat surface to tile on to. Whether it's smooth is neither here nor there.

[boltmail]

:

If you can't be bothered to read the refs, I can't be bothered to argue.

[mail]

Dear Peter

Bolt m has it right my tuppence hap'rth's worth is

have allowed for 12.5mm aqua panel

CONSIDER MERITS OF WEDI BOARD - EXPENSIVE (FEROCIOUSLY SO) BUT GOOD PRODUCT AND NOT MUCH NEEDED

+glue+tiles at the shower end and would appreciate some advice on what to do at the back and other side of the bath (I have not replaced a bathe for some years).

I intend to make my "chiseling" a bit more even by rendering the worst areas. Years ago I just used a cement render, GOOD PLAN TO MINIMISE RISK OF FAILURE AND MAXIMUM WATER RESISTANCE CONSIDER MERITS OF

3:1 SAND CEMENT (GOOD PORTLAND) INCORPORATE SIKA No 1 ADDITIVE ( IT IS SILICATIOUS NOT HYDROPHOBIC) AND SBR (NOT PVA WHICH IS RE-EMUSLIFIABLE) ALL AS PER M INSTRUCTIONS IE PAINT ON SBR FIRST APPLY RENDER WITH SBR IN IT YOU MAY NEED TO APPLY AS A SCUDDING OR SPLATTER DASH AND MAKE GOOD LATER WHEN SET BUT NOT HARD

has bonding/bonding plaster replaced this or should I still render? NOT IN MY BOOK - AS A GYPSUM PRODUCT IT IS INHERENTLY CHEMICALLY THE WRONG INGREDIENT

On the other walls I will need to have a base for tiling about 5-7mm thick, again should I be rendering or plastering? RENDER AS ABOVE I assume wall boarding is not a good idea near the bath! NOT UNLESS IT IS WEDI BUT LOOKS LIKE YOU DONT HAVE SPACE UNLESS YOU ARE PREPARED (AS IS GOOD DESIGN) TO HAVE THE WALL OVER THE BATH SIDE SO THE DEFAULT IS WATER TO FALL IN) IN WHICH CASE WEDI IS FINE

If bonding, do I need to also use a finishing coat, I was think of just tiling on the bonding? BEST TO TILE ON RENDER OR WEDI

Must you put a finishing coat onto dry or wet bonding (general question)? NO

Best wishes Chris

[Phil L]

You're going to 'slice' an extra 250mm from a 100mm breezeblock wall?

Aqua panel all round matey - with the adhesive it's about 16mm thick - it's a bit more pricey than rendering / bonding etc, but *much* quicker and you can tile straight onto the boards.

It doesn't matter how rough it is for drylining adhesive - I've seen this stuff used 3 inches thick at one side of a board and 3 mm thick at the other in order to level things up.

It's what aquapanel is made for, and it can be screwed on just like p-board if you have studding.

Tiling on bonding is a PITA - the adhesive goes off almost immediately, which means you'll have to skim, which is also a PITA when it's being tiled over

[nobodyhome]

I have seen some weird "quoting" on here but that takes the biscuit .

What Newsreader are you using and why use CAPITALS?

[Phil L]

I stopped reading it after the first bout of shouting, most google groups users tend to start off a bit 'iffy', but I see from GG that he's posted here at least 150 times before, but as you say, this one is a total mess.

[sm_jamieson]

If you douse the bonding in dilute PVA (it will soak in), it becomes a fine surface for tiling to. You should not tile onto raw bonding, as it will lack strength and suck the adhesive dry. The standard advice is certainly correct about not tiling onto raw bonding - consider that a skim typically becomes a hard "shell" on top of the softer undercoat plaster. But if you want to save money on skimming, I've found this to work fine, despite that fact that it may technically be a bodge. Of course you may need to do certain things to comply with the tile adhesive manufacturers warranty for commercial work etc. Also, I would not hang heavy porcelain tiles on plaster at all. Render or a cement board would be a safer choice. Simon.

[The Natural Philosopher]

It depends on the adhesive.

The standard advice

Bonding plaster I have used has been much harder than the skim finish plater.

Tiles do not hang: they are a very thin wall. The wall behind merely stops the wall buckling. the tiles and the grout are self supporting by and large.

I have tiled over almost everything up to and including expanding foam. The main requirement is rigidity, and flatnesss. Nothing else really matters apart from porosity. And that can be controlled with PVA.

Just use a good quality tile cement. I now swear by evostik waterproof.

[Peter Hemmings]

Sorry for the delay, I am about half way through the "25mm" off of my old breeze blocks! (sorry for the typo Phil).

What a bl**dy black mess I've got!

The Natural Philos>

I have come to conclusion that is summed up above. I had made the mistake of using inferior adhesive and I will also never buy "Cheap" tile adhesive again. I did try and tile onto dry bonding and witnessed at first hand the error of trying to get tiles onto a porous surface!

I understand that there are "approved" methods of tiling but like the above comments, I also have used the "not perfect" PVA solution (excuse pun) to obtain satisfactory results in the past.

FWIW I will be looking at using Aqua Panel for the bath side wall as I will almost certainly loose most of the very dry render that's under the "50's" tiles! It is alos ont to a coldish north wall and any form of slight insulation should help reduce condensation.

[Bolted]

In which case, use wediboard or equivalent like someone suggested, and not a cement panel which has f*ck all insulation value.

[The Natural Philosopher]

Not sure what wediboard is, but under tiles I'd be inclined to use a bit of stud, and something thinner and stringer - maybe 6-12mm MDF - and stuff polystyrene or celotex behind it. I've had good results tiling MDF. Make all my bath panels out of it.

[mail]

To revert to the points raised by Nemo - I did not quote - I responded all the answers are my own I am not reading any newspapers at present - far far too busy on this site

grovelling apologies to Nemo and any in the group who percieved my posting as shouting. It was not. It was simply an effort to make it easy for the reader to view the responses versus the question I am duly chided and will use colour or indentation in future.

On the issue of Aquapanel this is simply Portland cement held together by fibreglass and has some minor limitations viz it can absorb water it is heavy If it does get wet the glue does not take it requires Q4 at the joints to keep it dry It has a low U value

None of this applies to Wedi, hence my strong recommendation of it despite its price

there is nothing much "wrong" with it and the choice is, of course, yours, Peter Chris G

[mail]

Not "at least" ...in fact, it was 146 since I joined this group. Yours happily in lower case chris

[mail]

~Wedi board is Styrofoam* IBF, an HCFC blue extruded polystyrene foam with closed cellular structure and flame retardant additive. Technical data for those interested from the website

Compressive strength (50years)

[boltmail]

You should try wediboard then (or the likes, there are many makes, aquapanel *thermal* being one).

It's poly foam with a mesh reinforced polymer-modified thin (1mm-2mm) cement face on either side. It is waterproof (unlike aquapanel and most unlike MDF). It is very stiff (much, much stiffer than mdf). It is insulating. It is easy to work with, and purpose designed for tiling.

Fantastic stuff, and it has made normal aquapanel pretty redundant. It's only downside is that it is expensive.